1. Field of the Invention
The present invention relates to an interferometer that modulates a measurement light with a reference light to obtain a modulated light having a phase associated with movement of an object, and converts the modulated light into an electrical signal by a light-receiving device to obtain information of displacement of the object.
2. Description of the Related Art
Conventionally, a laser interferometer having high accuracy and high resolution is used as an apparatus for measuring the displacement of an object such as a stage. FIG. 7 is a view showing the arrangement of a conventional interferometer. This interferometer is disclosed in Japanese Patent Laid-Open No. 05-071913. A laser beam 11 emitted from a light source 1 and having a wavelength λ (633 nm) enters a polarization beam splitter (PBS) 2, and is split into reference light 12a and measurement light 12b by a PBS plane 2P. The reference light 12a is reflected by a reference mirror 4a and re-enters the PBS 2 through the same optical path. Since the reference light 12a is transmitted through a λ/4 plate 3a twice, the P wave is converted into the S wave. Therefore, the reference light 12a is transmitted through the PBS plane 2P this time, and enters a reflective element 5 as a light beam 13a. On the other hand, the measurement light 12b is reflected by a reflecting surface 4b, and re-enters the PBS 2 through the same optical path. Since the measurement light 12b is transmitted through a λ/4 plate 3b twice, the S wave is converted into the P wave. Accordingly, the measurement light 12b is reflected by the PBS plane 2P this time, and enters the reflective element 5 as a light beam 13b like the reference light 13a. After that, the reference light is retransmitted through the PBS 2 to form a light beam 14a, and the measurement light 12b is reflected by the PBS 2 again to form a light beam 14b. The light beams 14a and 14b are transmitted through the λ/4 plate twice, and re-enter the PBS 2. The light beams 14a and 14b are combined into a light beam 15, and an interference signal having a λ/4 period is obtained by the movement of the reflecting surface 4b. 
In the conventional interferometer shown in FIG. 7, a large number of reflecting surfaces exist in the optical paths. For example, a component reflected by a reflecting surface 21b of the λ/4 plate 3b propagates through the same optical path as that of ordinary measurement light. This component is finally superposed on the light beam 15, and modulated by the movement of the reflecting surface 4b. However, the component reflected by the reflecting surface 21b of the λ/4 plate 3b arrives at the reflecting surface 4b after being reflected only once. Therefore, the modulation amount is half that of the ordinary reflection component, and an interference signal having a period of λ/2 is obtained. Assuming that the reflectance of an antireflection coat (AR coat) of the reflecting surface 21b is 0.2%, the interference intensity of an interference signal generated from ghost light (stray light) reflected by the reflecting surface 21b is about 9% that of a main signal from the viewpoint of wave optics. Even when an AR coat having a very low reflectance of 0.01% is formed, an interference intensity of 2.5% is generated. A waveform as shown in FIG. 8 is obtained because sine-wave signals caused by all ghost light components are superposed on an electrical signal output from a light-receiving device 16. It is normally possible to obtain a sub-nanometer resolution by electrically dividing a sine-wave periodical signal modulated in accordance with the obtained displacement. However, deterioration of the linearity as shown in FIG. 9 occurs in the signal on which the components caused by the ghost light are superposed, and the error amount reaches a few nm to a few ten nm. This poses a serious problem in ultra-high-accuracy applications.